After updating to version 1.4 (on Windows 8.1) this happened: I have a folder is set to sync with another machine that I use as a backup. One of the files that is synced is a large Outlook PST (13 GB) that is updated several times a day as new email comes in. The file only syncs when Outlook is closed and releases the file. I had Outlook open for 3 days, during which time I updated BTSync from 1.3 to 1.4 on both machines. When I closed Outlook, BTSync started to sync as expected (copying the updated file to the backup machine which took several minutes). However, as the sync neared completion, it stopped and started syncing the file in the opposite direction for no apparent reason. I allowed it to finish and then tried to open Outlook. The PST file was corrupted and would not open. After repairing it, I was able to open it but discovered that it was missing all of the email from the last 3 days -- which means it had copied the old file from the backup machine over the newer one on my machine. The system time on each machine is nearly identical (only differs by a few seconds). I tried to duplicate the error but was not able to. The sync worked correctly on subsequent attempts to open and close Outlook. Perhaps the problem only occurs on the first sync attempt after updating to 1.4?